docs: document mcp server usage

docs/mcp.md

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+# metin-release-mcp
+
+Thin [Model Context Protocol](https://modelcontextprotocol.io/) server that
+wraps the Phase 1 `metin-release` CLI. Each `release …` subcommand is exposed
+as an MCP tool. The server contains no release business logic: it shells out
+to the real CLI with `--json` and returns the parsed envelope verbatim.
+
+## Install
+
+The server ships as an optional extra alongside the main CLI:
+
+```
+pip install -e '.[mcp]'
+```
+
+This installs the `mcp` Python SDK and adds a `metin-release-mcp` console
+script plus a `python -m metin_release_mcp` module entry.
+
+## Running
+
+The server speaks MCP over stdio, so you wire it into an MCP-capable client
+(Claude Desktop, Claude Code, etc.) as a stdio command. Example client entry:
+
+```json
+{
+  "mcpServers": {
+    "metin-release": {
+      "command": "metin-release-mcp",
+      "env": {
+        "METIN_RELEASE_BINARY": "/usr/local/bin/metin-release"
+      }
+    }
+  }
+}
+```
+
+You can also poke at it directly:
+
+- `metin-release-mcp --help` — list registered tools
+- `metin-release-mcp --list-tools` — dump the full tool JSON schemas
+- `metin-release-mcp --version`
+
+## Tools
+
+| Tool | CLI subcommand |
+|---|---|
+| `release_inspect` | `metin-release release inspect` |
+| `release_build_manifest` | `metin-release release build-manifest` |
+| `release_sign` | `metin-release release sign` |
+| `release_diff_remote` | `metin-release release diff-remote` |
+| `release_upload_blobs` | `metin-release release upload-blobs` |
+| `release_promote` | `metin-release release promote` |
+| `release_verify_public` | `metin-release release verify-public` |
+| `release_publish` | `metin-release release publish` |
+
+Tool input keys match CLI flag names with `_` instead of `-`
+(`--base-url` → `base_url`, `--dry-run` → `dry_run`). Boolean fields
+correspond to argparse `store_true` flags: pass `true` to set the flag,
+omit or pass `false` to leave it off.
+
+### Example invocation
+
+```json
+{
+  "name": "release_inspect",
+  "arguments": {
+    "source": "/srv/metin/client"
+  }
+}
+```
+
+The server runs `metin-release release inspect --json --source /srv/metin/client`
+and returns the full JSON envelope:
+
+```json
+{
+  "ok": true,
+  "command": "release inspect",
+  "status": "inspected",
+  "stats": {
+    "source_path": "/srv/metin/client",
+    "file_count": 9166,
+    "total_bytes": 3523473920,
+    "launcher_present": true,
+    "main_exe_present": true
+  }
+}
+```
+
+## CLI resolution
+
+On every tool call the server resolves the `metin-release` binary in this order:
+
+1. `METIN_RELEASE_BINARY` environment variable, if set and non-empty
+2. `shutil.which("metin-release")` against `PATH`
+
+If neither resolves, the tool call returns a wrapper-level error envelope
+(`error.code = "cli_not_found"`).
+
+## Error handling
+
+The server never invents its own release-level errors. There are three paths:
+
+- **Success** — CLI exits 0 with a valid JSON envelope → envelope returned as-is
+- **CLI-level failure** — CLI exits non-zero with an `{"ok": false, "error": …}`
+  envelope → that envelope is returned as-is, plus the CLI's stderr is
+  attached as a diagnostic text block
+- **Wrapper failure** — binary missing, unparseable stdout, unknown tool,
+  invalid input → synthetic envelope with one of
+  `cli_not_found`, `cli_unparseable_output`, `unknown_tool`,
+  `invalid_tool_input`
+
+## Environment variables
+
+| Variable | Purpose |
+|---|---|
+| `METIN_RELEASE_BINARY` | Override the `metin-release` binary path. |
+
+Any env vars the wrapped CLI honours (`METIN_RELEASE_MAKE_MANIFEST`,
+`METIN_RELEASE_SIGN_MANIFEST`) are inherited by the subprocess unchanged.